One apparatus of this type is described in French published patent specification No. 2541441. In this prior published French patent specification, the envelope in which the apparatus is installed is constituted by the well of a condenser in an electricity power station. The system described includes two pipes as defined above, one running along one wall of the tank and the other along the opposite wall. In addition, a third pipe fed with vapor at average pressure is situated at a level below said pipes and has vapor escaping therefrom via a row of holes. The top of the tank serves as an overflow from which liquid falls into the condenser well. Clearly in such an application, the fluid is generally H.sub.2 O, with the liquid and vapor states being constituted by water and steam respectively.
Unfortunately, this prior disposition does not give full satisfaction, and in particular it does not degas the fluid thoroughly enough.